The invention relates to a large scale technical process for sulfuric acid treatment of clays or aluminiferous minerals for the production of pure aluminum oxide via a basic intermediate.
The known methods for the production of aluminum oxide generally comprise the steps of treatment of Al.sub.2 O.sub.3 - containing minerals with sulfuric acid under pressure and at elevated temperatures, reduction of the iron in this solution in order to avoid its precipitation, hydrolysis of the aluminum sulfate solution and thermal decomposition of the precipitated basic aluminum sulfate (BAS).
In this context it is also recognized that the yield of solid hydrolysis product is very strongly dependent upon the basicity of the solution, expressed for example by the ratio of SO.sub.3 to Al.sub.2 O.sub.3. In order to achieve the lowest possible ratio, the addition of basic aluminum sulfate or active alumina has been suggested. For this purpose, either the basic sulfate produced during the pressure application step is employed, or else an alumina which remains active during the thermal decomposition is added. Through this method of correction, according to these proposals a substantial amount of basic sulfate or active alumina is recirculated through the system, inducing a substantial burdening on the system. This results in further energy and apparatus outlay, and the accompanying increase in costs.
An alternative method for increasing the basicity of the solution lies in the addition of alkali metal hydroxides or salts. Over a longer period of time, the addition of these chemicals leads to an inpermissible contamination of the product. In order to obtain a product of quality suitable for fusion electrolysis, corresponding purification steps are required, which again increase the apparatus and energy costs of the overall process.
Thus, it is clear that the known methods for sulfuric acid treatment of clays or aluminiferous minerals and the subsequent steps to the preparation of aluminum oxide are economically unfavorable because of high energy and apparatus costs. For this reason, these methods have not found large scale technical application. The recirculation of materials just as a necessary purification of the product are major disadvantages of the known procedures.